Russell asked this question: There was a barber in a village who promised to shave all the people in the village who didn't want to shave their heads. So, as promised, will he shave himself?

Suppose he is willing to shave, he can't shave himself as promised; On the other hand, if he doesn't want to shave, he must fulfill his promise to shave. This is Russell's paradox, which led to the third mathematical crisis.

After the analysis of several algebras and the application of various logical reasoning methods, mathematicians all over the world finally reached the understanding that this problem can never be solved, so the third mathematical crisis was solved.

About the fourth mathematical crisis, it is entirely possible. As for the specific situation, it is difficult to predict, because mathematics is becoming more and more theoretical, and its loopholes are difficult to find in practice.

Judging from the previous three crises, the direct cause is the emergence of new paradoxes. Therefore, the fourth crisis may still be triggered by public opinion.